1. Field of the Invention
The present invention relates to a photovoltaic device, and, more particularly to a photovoltaic device such as a solar cell or the like which uses amorphous silicon (hereinafter abbreviated to "a-Si:H").
A photovoltaic device of the type described above can be suitably used as a power source, and more particularly as a power source for electricity for use in various electronic devices and power devices.
2. Related Background Art
The study of applying a-Si:H to a photovoltaic device such as a solar cell started with the invention of a photovoltaic device by D. E. Carlson (U.S. Pat. No. 4,064,521) on the basis of a succession of doping processes (Solid State Communications, Vol. 17, pp. 1193-1196, 1975) discovered by W. E. Spear and P. G. Le Comber.
Recently, photovoltaic devices having a-Si:H have been used in clocks, compact calculators, outdoor lamps, etc. However, problems of low conversion efficiency and deterioration in the characteristics with time arise if the a-Si:H photovoltaic device is used for generating electricity. Accordingly, various studies have been made in order to further improve the conversion efficiency of the a-Si:H photovoltaic device and to prevent deterioration in the characteristics of the same with time.
Hitherto, occurrence of a short circuit in the photovoltaic device has been prevented by forming a transparent conductive layer between a conductive substrate and the a-Si:H. Furthermore, the conversion efficiency has been improved by utilizing the reflection enhancing effect (Japanese Patent Publication Patent No. 59-43101). However, although forming of the transparent conductive layer will improve the conversion efficiency, a satisfactory effect of preventing deterioration in the characteristics cannot be obtained. In particular, in a case where the photovoltaic device is used for generating electric power, there is a desire of preventing the deterioration in the characteristics. Since the deterioration in the characteristics is mainly due to light irradiation, it is a critical problem when the photovoltaic device is used.
Hitherto, a first conductivity type layer has been considered to be a part of the photoelectric conversion portion of the photovoltaic device. Therefore, it has been considered that modification of the first conductivity type layer will improve the conversion efficiency of the photoelectric conversion portion and prevent deterioration. However, it is too complicated and difficult to improve the conversion efficiency of only the photoelectric conversion portion and to prevent deterioration in the characteristics.